1. Field of the Invention
The present invention relates to a sheet processing apparatus and an image forming system, and more particularly to a sheet processing apparatus provided with a function for folding a sheet-like recording medium such as a paper sheet, a recording paper sheet, and a transfer paper sheet (hereinafter, simply referred to as a “sheet”), and an image forming system provided with the sheet processing apparatus.
2. Description of the Related Art
Some post-processing apparatuses used in combination with an image forming apparatus such as a copier have conventionally been capable of binding one or more sheets into a saddle stitched brochure by binding the sheets at the center and folding the sheet bundle at the center with a folding roller pair provided in parallel with a direction in which the sheets are folded. Also known to enhance the fold line portion on a saddle stitched brochure is a technology that performs additional folding to the brochure using a roller carried along the spine of the brochure.
In such a technology providing additional folding, to provide additional folding along the spine (fold line portion) of a brochure (sheet bundle) using an additional folding roller, the roller is kept standby outside of the brochure, placed on top of the spine of the brochure, and then carried along the spine.
As an example of the additional folding technology, the invention disclosed in Japanese Laid-open Patent Publication No. 2009-190824 is known. This invention discloses a sheet folding apparatus that is provided with an additional folding unit including a pressing roller that rolls along the fold line portion of a sheet bundle that is folded by a folding unit, an elastic biasing unit that elastically applies a biasing force to the pressing roller in the thickness direction of the sheet bundle, and a driving unit that carries the pressing roller in a direction perpendicular to the conveying direction of the sheet bundle. The sheet folding apparatus provides additional folding by causing the additional folding unit to press the fold line portion of the sheet bundle having already folded by the folding unit. The sheet folding apparatus is characterized in including a lifting unit that brings up the pressing roller before the pressing roller reaches an end surface of the sheet bundle so that the pressing roller is not allowed to roll over the end surface, and then lowers the pressing roller from a predetermined position, whereby bringing the pressing roller into contact with the top surface of the fold line portion.
In the invention disclosed in Japanese Laid-open Patent Publication No. 2009-190824, guiding members are used to bring up the pressing roller before the pressing roller reaches the end surface of the sheet bundle, and the pressing roller is caused to press the sheet bundle from the top of the fold line portion of the sheet bundle again. Two slide shafts are used in moving the roller.
In this manner, in the invention disclosed in Japanese Laid-open Patent Publication No. 2009-190824, guiding shafts serving as guiding members are used in moving the pressing roller rolling along the spine (fold line portion) of the sheet bundle. In such a structure, if a high load is applied to the pressing roller to enhance the fold on the sheet bundle, the slide shafts might become bent. If the slide shafts become bent, the pressure becomes insufficient at the center of the slide shafts. Therefore, it has been difficult to stabilize the pressure applied to the sheet bundle.
In view of the above-mentioned problems on the conventional art, there is need to stabilize the pressure applied to a saddle-stitched sheet bundle when additional folding is applied along the spine of the sheet bundle.